This invention relates to a device for indicating insufficient inflation pressure in vehicle tyres.
Known devices comprise substantially a transducer housed in a wheel of the vehicle, and arranged to open or close an electrical contact if the tyre pressure falls below a set minimum value. The state (open or closed) of the electrical contact, and consequently the state of the tyre pressure, is transmitted electromagnetically or by inductive coupling, to a processing circuit disposed on the vehicle body and arranged to indicate insufficient pressure by means of optical and/or acoustic indicators.
The devices which transmit this state electromagnetically require the use of a transmitter circuit (for example an oscillator) mounted on each wheel and supplied by an associated battery. These have the advantage of allowing indication under any wheel condition (whether at rest or rotating), but have a serious drawback related to the supply battery, which requires frequent checking of its state of charge, and puts the wheel out of balance. As a transmitter circuit with its own battery is required for each wheel, the total cost of a device of this type is fairly high. Finally, because of the type of transmission system used, these devices can cause electromagnetic disturbances, and can be influenced by electromagnetic fields external to the vehicle.
Devices which transmit the state of the electrical contact by inductive coupling comprise substantially a first and a second transfer coil housed in the wheel and connected electrically together so as to have a first terminal in common, with their second terminal being connected together via the electrical contact operated by said transducer. In proximity to each wheel, the vehicle body also houses an emitter coil connected to the output of a transmitter circuit, and a receiver coil connected to the input of a processing circuit and arranged to be coupled to the emitter coil only through said transfer coils, when said electrical contact is closed. An electrical signal therefore reaches each processing circuit from the transmitter circuit, and generally is of the periodic pulse type as it is received for each revolution of the wheel when the emitter and receiver coils are facing the first and second transfer coil respectively.
The tyre pressure can therefore only be checked when the vehicle is moving. This check is generally made by comparing the signals received from the various receiver coils with those provided by a receiver coil mounted on a reference wheel, and the lack of any signal from a wheel compared with the reference wheel gives rise to immediate indication of insufficient pressure.
The lack of a signal can however occur for reasons independent of the tyre pressure. For example, it can be due to recoiling of the transducer electrical contact caused by a particularly rough road, or can be due to a narrow bend travelled by the vehicle, in which the inner wheels travel through a smaller space than the outer wheels and therefore emit a smaller number of electrical signals than these latter.
Finally, when the vehicle is at rest, for example at traffic lights, there is generally a complete lack of signals from the receiver coils, with a consequent immediate indication of insufficient pressure in all the vehicle wheels.